1. Field of the Invention
The present invention relates to a radiation thermometer for measuring temperature of a target object, and more particularly it relates to an optical system for such a radiation thermometer including a viewfinder for sighting the thermometer such that its optical system for the measurement (hereinafter referred to as a measuring optical system) is correctly aimed at and focused on a target object to be measured.
2. Description of the Prior Art
Various kinds or types have been known for the aforementioned optical system, and most of them employ a single lens reflex type viewfinder of which the optical axis coincides with the optical axis of the measuring optical axis and with which parallax does not occur between the measuring and viewfinder optical systems. FIG. 1(a) shows an exemplary one of the conventional optical systems for a radiation thermometer for measuring temperature of the range not less than 100.degree. C. With reference to FIG. 1(a), the light from a target object to be measured passes through the off-axis portion of a lens L1 and is focused on a photoelectric detector 2 of which the output is processed to present the temperature of the target object. The light beam having passed through the central portion of the lens L1 is reflected by a mirror 4 and directed out of the measuring light path area to form an image of the target object through or by means of a viewfinder optical system including a mirror 6, a focusing plate 8 and an eyepiece 10. The lens L1 can move along the optical axis for the focusing. Hence, the viewfinder enables the observation of the image of the target object being measured by the radiation thermometer and of the focusing condition of its optical system. FIG. 1(b) shows another optical system wherein a beam splitting mirror 12 consisting of a dichroic mirror or a half-mirror is used instead of the mirror 4 in FIG. 1(a). Those constructions require less components and are simple. However, in order to enable measurement of the temperature to the ordinary temperature range with those constructions, the lens L1 must be made of a material that is transparent to the light of the long wavelength range of 8-14 .mu.m as well as of the visible light wavelength range. Diamond has such an optical property but is very expensive. NaCl having also such a property is unstable both physically and chemically.
Accordingly, for the optical system of the radiation thermometer, one may thihk of a combination of a measuring optical system of the Cassegrainian type and a viewfinder, as shown in FIG. 2a and 2b. With reference to FIG. 2a, the light to be measured is reflected at the peripheral portion of the main mirror 14 of a Cassegrainian type optical system and then at the peripheral portion of a submirror 16, and passes through the aperture 14a of the main mirror 14 to impinge on the photoelectric detector 2. On the other hand, the paraxial light beam traveling in the Cassegrainian type optical system is reflected by mirrors 18 and 20 respectively and is introduced through a viewfinder lens L.sub.F, a focusing plate 8 and eyepiece 10 to the eye of the user or observer enabling the observation of the object image. FIG. 2(b) show a modification wherein the viewfinder lens L.sub.F is disposed in front of the mirror 18 with its optical axis coinciding with the optical axis of the Cassegrainian type optical system, the mirror 20 of FIG. 2(a) was substituted by a pentagonal prism 22 for inverting the image, and further a condenser lens 24 is interpose between the focusing plate 8 and the eyepiece 10. In front of the photoelectric detector 2 is disposed a pin-hole plate 26 for restricting the Width of the light beam incident on the detector 2. The constructions as shown in FIGS. 2(a) and 2(b) dispenses with the lens in the measuring optical system and accordingly does not require the expensive material or the physically and chemically unstable material so that they will not be expensive nor physically and chemically unstable. However, they will be cumbersome and lose compactness since the mirror 18 must be disposed in front of the submirror 16. Additionally, in the case of the example shown in FIG. 2(a), since the viewfinder lens L.sub.F is distant from the mirror 18, the viewfinder image will not be observed if the user shifts his or her eye by a little amount. The example shown in FIG. 2(b) is free from this disadvantage but is more cumbersome because not only the mirror118 but also the viewfinder lens L.sub.F are disposed in front of the submirror.